1. Field of the Invention
The invention relates to an apparatus for and method of measuring the composition and the pressure of the discharged gas from a hot cathode ion gauge mounted to a container of an ultra high vacuum (<10−7 Pa) using a residual gas analyzer.
2. Background of the Related Art
In general, an ion gauge employed in the high-tech display or semiconductor fabrication process is a pressure sensor, which is used in the measurement of the pressure in high vacuum or ultra high vacuum.
FIG. 1 is a schematic cross-sectional view showing the principle of a hot cathode ion gauge used in the present invention. As shown in FIG. 1, a main body 106 is formed of transparent glass or metal material, and a filament 104 is provided at the inside thereof.
A vacuum port 112 is a constitutional portion to be connected to a pressure container 210, and an ion collector 108 is hanged by extension from the upper portion of the center thereof.
A filament 104 is provided at one lower side of the main body 106, and a grid electrode wire 110 is provided at the other lower side. Also, a third direct current source 140 and a variable resistor 142 are respectively connected to both ends of the filament 104, and a second direct current source 144 with 175V is connected to the electrode wire 110. A first direct current source 146 with 25V is connected between the variable resistor 142 and the second direct current source 144.
An amplifier 120 is connected to an ion collector 108, and a pressure gauge 125 is connected to the amplifier 120 in series.
Hereinafter, the principle of measuring the pressure of the gas in accordance with the ion gauge 100 constructed as above will be described below.
When the vacuum is generated in the ion gauge 100 through the vacuum port 112, a very rare number of gas molecules are existed in the ion gauge 100.
In this instance, thermo-electrons 114 discharged by the current flowing through the filament 104 collide with the gas molecules to thereby produce ions 118. The produced ions 118 are collected in the ion collector 108 to thereby generate minute electric current in proportion as the number of the ions collected. The amplifier 120 reacts to amplify minute electric current outputted as described above, and the pressure gauge 125 denotes the inside pressure of the ion gauge 100 in proportion as the amplified electric current signal.
With regard to the conventional hot cathode ion gauge, several problems were existed as follows. In other words, when the ion gauge 100 is under the ultra high vacuum (<10−7 Pa), there occurred a phenomenon of the discharge of the gas from the ion gauge 100 itself. The gas discharged from the ion gauge 100 causes grounds of contaminating the vacuum container or increasing the pressure, so that it was difficult to measure the vacuum pressure accurately.
By this time, it was impossible to anticipate the effect of the gas, because it was impossible to know the kind and volume of the gas (i.e., pressure) discharged from the hot cathode ion gauge quantitatively. Accordingly, it was impossible to anticipate the effect of the contaminated process caused by the discharged gas of the ion gauge 100 on the quality and durability of the products.
Especially, because the gas discharged from the ion gauge, which was indispensable to measure the ultra high vacuum in the high-tech vacuum processes such as those employed in the fabrication of the semiconductor or the display, reacted with other gas in the process or increased the pressure of the gas, so that these gases caused grounds of reducing the capacity or the durability of the products.